1. Field of the Invention
The present invention relates generally to stencil sheets, and more particularly to such sheets or blanks which are sensitive to the production of a stencil record by mechanical force.
2. Prior Art
Stencil sheets for use in image transfer and reproduction are well known. The stencil sheets used for duplicating generally consist of a special porous tissue paper which is coated with an ink-impervious substance which may be displaced by pressure such as that applied by a typewriter key or stylus. The areas of the stencil sheet where the ink-impervious substance or coating has been displaced permit the passage of the ink, and allow the stencil to be used in a printing process to transcribe the information thus inscribed.
The majority of the stencil sheets presently in use are prepared by applying a solvent solution coating to a thin tissue-like paper with the coating being dried on the paper as the result of solvent vaporization. A wide varity of materials have been proposed and used to constitute the coating, including cellulose nitrate, cellulose ethers and esters, synthetic resins, rubber compounds and even metal films. Both the natural and synthetic resin coatings have conventionally applied by dispersion in a low-boiling solvent system that requires, upon application to the tissue base, the above noted solvent vaporization.
The most popular film forming component in stencil coatings remains cellulose nitrate, as it offers better coating quality at a reduced raw material cost, however the use of low-boiling solvents has remained a primary detractor. In particular, the time required for volatization of the solvents from the applied coating materially increases the total production time necessary to form the finished stencil product. Additionally, the low flash points of the solvents presently in use renders the stencil coating operation hazardous and expensive, as additional equipment is necessary to provide adequate fire prevention and to achieve solvent recovery. Moreover, the discharge of solvents into the atmosphere represents a source of air pollution which is violative of present pertinent Federal Regulations, and is costly to abate.
As noted earlier, other coatings have been investigated which rely upon variant methods of applications, such as the so-called "hot-melt" process, however these coatings have met with limited commercial acceptance and success.